In the field of electronically controlled fuel injection systems, it is imperative that electromagnetic solenoids be provided which are capable of high speed operation and have consistently reproducible stroke characteristics. The necessity of high speed operation requires little explanation when one considers that an engine operating at 2000 rpm must have fuel injected into each cylinder of a multicylinder engine at 10 millisecond intervals and the entire injection pulse occurs over only a 1 millisecond period. Slow acting solenoids result in erroneous quantities of fuel being delivered to each cylinder at an inappropriate timing advance which can adversely affect the performance of the engine.
High speed solenoid operation is obviously an absolute necessity; however, the need for consistently reproducible stroke characteristics is a less obvious but equally important requirement. A reproducible solenoid stroke provides the precise control needed to obtain maximum fuel efficiency, power output, and engine life and has also been shown to have beneficial effects on the quantity and type of exhaust emissions. These benefits extend from the fact that the quantity of fuel injected into a cylinder is typically controlled by the duration of time for which the solenoid is maintained in an open configuration. Thus, a given voltage applied to the solenoid for a given duration of time should result in the solenoid being operated to an open configuration for a substantially standard duration of time and thereby deliver a standard preselected quantity of fuel. Once the relationship between voltage, time, and quantity of fuel has been established, it should remain constant throughout the useful life of the apparatus. Therefore, a fuel injection solenoid control can provide advantageous control of engine operation over the entire range of engine speed by delivering a regulated voltage for a variable duration of time.
Further, in the operation of a fuel injection system on a multicylinder engine, a fuel injection solenoid is provided for each engine cylinder and must be energized and de-energized for each compression stroke of the corresponding engine cylinder. Typically, the energy stored in the solenoid is transformed into heat by a diode and resistor combination placed in the flyback current path of each solenoid. The magnitude of the energy disposed of in this manner is significant and directly results in an increase to the cost of the system. The heat generated by the discharging solenoids exacerbates the problem of heat dissipation in an already thermally hostile environment. Additional means must be provided to remove the excess heat to maintain the reliability of the electronic hardware. Increased heat dissipation capability is a directly measurable cost.
Additionally, significantly greater power generating capability is necessary than would be if a portion of the stored energy could be recovered.
The present invention is directed to overcoming one or more of the problems as set forth above.